fix: check buffer validity in scheduled callbacks

It's possible for the buffer an autocommand triggers on to be wiped out
later in the same event loop.

lua/lspconfig/configs.lua

@@ -105,7 +105,9 @@ function configs.__newindex(t, config_name, config_def)
           -- Use vim.schedule() to ensure filetype detection happens first.
           -- Sometimes, BufNewFile triggers before 'filetype' is set.
           vim.schedule(function()
-            M.manager:try_add(opt.buf)
+            if api.nvim_buf_is_valid(opt.buf) then
+              M.manager:try_add(opt.buf)
+            end
           end)
         end,
         group = lsp_group,
@@ -153,7 +155,9 @@ function configs.__newindex(t, config_name, config_def)
               -- Use vim.schedule() to ensure filetype detection happens first.
               -- Sometimes, BufNewFile triggers before 'filetype' is set.
               vim.schedule(function()
-                M.manager:try_add_wrapper(arg.buf, root_dir)
+                if api.nvim_buf_is_valid(arg.buf) then
+                  M.manager:try_add_wrapper(arg.buf, root_dir)
+                end
               end)
             end,
             group = lsp_group,